1. Field of the Invention
This invention relates to the field of information storage and retrieval and, more particularly, to the recording of binary data in a magnetic medium.
The invention is directed to track-sector formatted binary recording on rotating magnetic disks. A system is shown which automatically reads both single and double density rate recordings. This system will read disks recorded entirely at the so-called "single" density rate, entirely at the so-called "double" density rate or partially at single density and partly at double density.
2. Prior Art
Various systems are known in the prior art for recording binary data on magnetic media. The recording of binary data on rotating magnetic disks using a sector-track format is conventional. In such systems, magnetic read/write heads are used to transfer data onto and off of the rotating disk. The disk is divided into a number of concentric tracks each having a plurality of distinct segments or sectors comprising identifiable regions where data may be recorded. Information is stored on the disk as a series of magnetic flux reversals, at normally specific positions within the sectors. Accurate positional synchronization of the read and write operations is therefore required for accurately extracting information from the flux reversals. For this reason, a "clock" or synchronizing signal is recorded together with the data. In the reading operation, a phase locked loop (PLL) is employed to detect and lock onto the synchronizing signal, to generate an internal time-base reference so that desired data may be read from the correct position on the disk.
Prior disk readers have been able to read and decode (on any one pass of the disk) at only a single recording density for all data fields on the disk,--e.g., either single or double density recordings. None of the prior art disk recording systems have, to my knowledge, provided the capability of reading disks which are recorded at a first density on a first sector and recorded at a second density, on a second sector without requiring two passes (i.e., one for each density rate).
This is due at least in part to the problems caused by bit shift. Due to the interaction of the closely spaced magnetic fields arranged on the recording tracks of a disk, a binary symbol, e.g., a binary "1", may actually be recorded on the disk at a position slightly different from the one at which the recording head is instructed to write the information. For example, the flux reversals corresponding to two adjacent "1's" may tend either to repel or attract each other. This effect is known as "bit shift." A disk recording system, to provide reliable data recovery, must be able to accommodate bit shift. Even when a phase locked loop is used to track the recorded information during the read operation, bit shift can cause problems. Primarily, an excessive amount of bit shift will cause the PLL to lose phase lock. In the past, the effect of bit shift has been minimized by a technique known as pre-compensation. That is, for each bit the amount of bit shift is predicted (from an examination of the bit pattern of the data being recorded) and the recording system is adjusts or compensates therefor. Thus the recording head is provided with a signal to be recorded when it is at a position which is shifted from the nominal bit recording position by an amount equal to and oppositely directed from the predicted bit shift; theoretically, the bit is thereby written in the nominal position it was actually intended to occupy, as if there had been no bit shift. To perform this operation, prior art PLL disk reading systems have required complex pre-compensation circuitry. And, even with such pre-compensation circuitry, some bit shift will still occur; and excessive bit shift will therefore still cause the PLL to lose lock. Reducing the gain of the PLL increases its ability to maintain lock in the presence of a greater amount of bit shift, by increasing its lock-in range. As used herein, lock-in refers to the range over which phase lock is maintained by the PLL after having been initially acquired. However, decreased loop gain also increases acquisition time--the time that the loop requires for phase locking on the data being tracked. Yet acquisition time must be limited, since the number of bits available for acquisition is limited, to make efficient use of the storage medium. In prior art designs, therefore, PLL gain has been constrained to fall within a limited range in order to provide both rapid acquisition and a capture range adequate to accommodate at least modest amounts of bit shift. These constraints have thus far been obstacles to the design of a practical disk reader with PLL dynamic performance adequate to automatically accommodate disks having data fields recorded at both single and double density rates.
Accordingly, it is an object of the present invention to provide a reader for data recorded on magnetic disks at both single and double densities, including disks having some sectors recorded at the single density rate and other sectors at the double density rate.
It is another object of the present invention to provide a PLL for a magnetic disk reader capable of reading data recorded at both single and double density rates which obviates the need for pre-compensation to avoid bit shift.
It is a further object of the present invention to provide, in a magnetic disk reader, a phase locked loop having an automatically adjustable loop gain responsive to the function being performed by the loop (i.e., operation in acquisition or lock-in tracking mode).
Yet another object of the present invention is the provision of a magnetic disk recorder employing a phase locked loop having a short acquisition time together with a wide capture range.
Still other and further objects of the present invention will be apparent to those skilled in the art from the description of the present invention provided herein.